There has been a constant interest in the research on luminescent materials for OLED applications, whether to find suitable dopant material or to optimize the device structure to effect high quantum efficiency, long device lifetime or color tuning. Ir(III) and Pt(II) complexes dominate the research field. They are highly regarded for the outstanding luminescence quantum yield and stability.
To have these desirable properties, the structural design of the ligand apart from the choice of metal ion plays the determining role. A common practice is to use multidentate cyclometalating ligands. These multidentate ligands usually contain strong field donor atoms like C— or O— to remove the low-lying non-emissive metal-centered (MC) excited states. At the same time there is chelate effect endowed by multidentate ligands instead of monodentate ligands, which could render thermal stability to the complex.
Over the past two decades, the research in luminescent gold(III) compounds has surged. Luminescent Au(II) complexes exist. But the structural design relies heavily on cyclometallated tridentate complexes accompanied by an ancillary strong σ-donor, usually a carbanion. Only this kind of gold(III) complexes have been utilized in OLEDs devices. Relatively small number of high performance devices have been realized though the solution quantum efficiencies of these complexes. Nevertheless, the long term stability of these OLEDs may be affected by the relatively weak binding force between the gold(III) center and the auxiliary monodentate ligand.